With recent advances in nanoelectronics, products are being developed which apply physical phenomena unique to magnetic materials with minute sizes. Of these, there have been particularly rapid advances in the field of spin electronics, which utilize the spin of free electrons in magnetic materials.
In the field of spin electronics, spin valve elements utilizing the tunneling magneto-resistance (TMR) effect occurring in a layered structure of a ferromagnetic layer, an insulating layer, and a ferromagnetic layer in order, or utilizing the giant magneto-resistance (GMR) effect occurring in a layered structure of a ferromagnetic layer, nonmagnetic layer (conducting layer), and a ferromagnetic layer in order, are currently regarded as having the greatest possibility of application.
FIG. 5 and FIG. 6 are cross-sectional views showing the configuration of spin valve elements of the prior art. Of these, FIG. 5 shows the basic constituent portions of a spin valve element utilizing TMR. This element has a configuration in which an insulating layer 24, and a ferromagnetic layer 23 (fixed layer) and ferromagnetic layer 25 (free layer) sandwiching the insulating layer, are formed on a substrate 5; to this are further added, as necessary, electrode layers 21, 27, an antiferromagnetic layer (pinning layer) 22, a capping layer 26, and similar. The direction of the magnetization of the fixed layer 23 is fixed by magnetic coupling with the antiferromagnetic layer 22 and similar. When electrons are passed from the fixed layer 23 toward the free layer 25 in this element, a torque acts to cause the magnetization of the free layer 25 to be aligned parallel to the direction of the magnetization of the fixed layer 23. Conversely, when electrons are passed from the free layer 25 toward the fixed layer 23, a torque acts on the magnetization of the free layer 25 so as to be antiparallel to the direction of the magnetization of the fixed layer 23. Consequently depending on the direction of the current in the free layer 25, the direction of magnetization of the free layer 25 can be controlled. This phenomenon of magnetization inversion by electron spin is called spin transfer magnetization reversal. For reasons described below, in conventional structures the size in in-plane directions of the magnetic layers in such an elements must be kept to very small sizes (approximately 150 nm or less), and expensive equipment such as electron beam exposure equipment is used.
In order to suppress the exchange coupling due to the leakage magnetic field from the film edge portions of the ferromagnetic layers 23 (fixed layer) and 25 (free layer) sandwiching the insulating layer 24, generally the portion on the side above the insulating layer 24 is made sufficiently smaller than that on the substrate side, and an insulating film 30 is formed on the periphery thereof. A number of methods may be used to form these structures; for example, after forming the layered film from the substrate to the electrode 27, applying a negative resist and performing exposure using a photolithography method, ion milling can be performed to expose the upper portion of the insulating layer 24, and thereafter an insulating layer 30 can be formed by covering with SiO2 or similar, and after liftoff the electrode 27 to be used for wiring can be formed.
FIG. 6 shows the basic constituent portions of a spin valve element utilizing GMR. The difference between the element utilizing TMR of FIG. 5 and the element utilizing GMR is the replacement of the insulating layer 24 with a nonmagnetic layer 51; otherwise the configuration is basically the same.
Magnetic random access memory (MRAM) is attracting the most attention as an application of these technologies, and is drawing interest as a replacement for conventional DRAM (dynamic random access memory) and SRAM (synchronous DRAM).
Further, it is known that when an electric current and an external magnetic field are simultaneously applied to these spin valve elements, microwave oscillation occurs (see, for example, S. I. Kiselev, et al, “Microwave oscillations of a nanomagnet driven by a spin-polarized current”, Nature, Vol. 425, p. 380 (2003)). For example with respect to the current direction, suppose that a current is passed such that the torque acts on the magnetization of the free layer 25 so as to become antiparallel to the direction of the magnetization of the fixed layer 23, and suppose that an external magnetic field causes a torque to act on the magnetization of the free layer 25 so as to become parallel to the direction of the magnetization of the fixed layer 23. In this case, under conditions in which the two torques are counterbalanced, high-frequency oscillation in the microwave region can be induced.
In addition, it has been reported that when two elements are formed adjacently and when currents and external magnetic fields appropriate to these are applied, the oscillation frequencies and phases of the two become coincident, the frequency width is decreased, and microwave output at this time is also increased (see, for example, Kaka, et al, “Mutual phase-locking of microwave spin torque nano-oscillators”, Nature, Vol. 437, p. 389 (2005; F. B. Mancoff, et al, “Phase-locking in double-point-contact spin-transfer devices”, Nature, Vol. 437, p. 393 (2005); J. Grollier, et al, “Synchronization of spin-transfer oscillator driven by stimulated microwave currents”, Physical Review B73, p. 060409 (2006)). This phenomenon is called a phase locking phenomenon, and the mechanism, though not yet be clarified, is inferred to arise from interaction between the high-frequency magnetic fields occurring in each of the elements; this phenomenon is attracting attention as means of increased output.
In numerous reports, the oscillation output of the above microwave oscillator element stops at approximately 0.16 μW for TMR and at approximately 10 pW for GMR, which are very low levels for practical application. The simplest means to increase output is to increase the element area, but this is difficult for the following reason. That is, in spin valve elements, in order to facilitate coherent rotation of spins necessary for spin-transfer magnetization inversion, the magnetic films must comprise a single magnetic domain. For example, in order to obtain a single magnetic domain in the magnetic film the periphery of which is circumscribed on the left and right by the insulating film 30 in FIG. 5 and FIG. 6, the size circumscribed by the insulating film 30 on the left and right must be made small. In this way, the size of the element is required to be at most a size in which domain walls do not exist; although varying with material and shape, this size is approximately 150 nm. The size of a single conventional spin valve element cannot be made larger than this dimension.
Because there is an upper limit to the size of one spin valve element, in order to obtain a large output, numerous minute elements must be integrated. As means of integration, photolithography techniques are most widely used and have high precision; but in order to fabricate magnetic members with microminiature sizes (approximately 150 nm or less), investment in electron beam exposure and similar expensive equipment is necessary, so that there is the problem of high manufacturing costs.
Further, in “High-regularity metal nano-hole array based on anodic oxidized alumina”, control of the size, pitch and depth of porous holes when manufacturing a porous alumina film from an aluminum film through manipulation of external conditions is disclosed (see H. Masuda, “High-regularity metal nano-hole array based on anodic oxidized alumina”, Kotai Butsuri, Vol. 31, No. 5, p. 493, 1996). And, in “Nanoscopic templates using self-assembled cylindrical diblock co-polymers for patterned media”, an invention is disclosed which is intended for applications in so-called bit-patterned media of hard disks (see X. M. Yang, et al, “Nanoscopic templates using self-assembled cylindrical diblock co-polymers for patterned media”, J. Vac. Sci. Technol. B, Vol. 22, p. 3331 (2004)).
However, when numerous elements such as described above are connected in parallel, the overall impedance of the integrated element declines with the number of elements, that is, with increasing total area of the elements. However, in general in high-frequency circuits it is necessary to match impedances in order to suppress transmission losses. In the microwave region, generally input and output impedances are set to be 50Ω. Even when there are such input/output impedance settings, the oscillation output can be increased even when elements are connected in parallel as described above, but measures must be taken with respect to the overall electrical resistance, which declines as the number of elements increases. Further, there is the problem that if magnetic elements are simply connected in parallel, synchronized oscillation may not occur between these elements.
Further, in addition to parallel connection of spin valve elements, the oscillation output can also be increased through series connections. In the case of series connections, as the number of elements increases the overall electrical resistance increases. In order to use spin valve elements in series connections, a method in which photolithography is used to form numerous planar separate elements which are connected by separate wiring, and a method in which a separate substrate is used for wiring to connect elements, are conceivable. However, in methods in which numerous separate planar elements are formed by electron beam exposure or other means in widespread use, there is the same problem as in the prior art of the need to invest in expensive equipment, and in a method in which elements formed on a separate substrate are connected by wiring, there are such practical problems as an increase in the number of processes and limitations on the number of connected elements arising from limitations on the element dimensions.
In light of these circumstances, a low-cost method is sought for adjustment the element impedance and suppressing transmission losses of the oscillation microwaves.
Moreover, when spin valve elements are manufactured using porous film, there is an upper limit to the sizes of satisfactory porous films which can be obtained, and formation of spin valve elements of arbitrary area using porous film is difficult. Consequently even when a lower impedance is desired, broadening the spin valve element area to adjust the impedance is difficult.